The present invention relates to an amplifier device intended for incorporation in a cable of a cable television distribution network, comprising two parallel paths each having their ends interconnected for forming an input/output terminal, the first path being provided with an amplifier operating in a first frequency band, the second path being provided with an amplifier operating in a direction opposite to that of the first amplifier and in a second frequency band other than the first band, having taken in a direction from each terminal a first filter element having a characteristic of a first type for filtering the signals of the first path and a second filter element having a characteristic of a second type complementary to the first type for filtering the signals of the second path.
Devices of such a type are inter alia used in known cable television distribution systems such as "MATV" or "CATV", depending on whether the distribution is inside a building or for a town.
In such a device, the amplifiers are loopwise connected to each other and the resultant feedback will cause the assembly to oscillate if adequate precautions are not taken; the presence of filter elements which transfer passbands to each amplifier independently of each other render it possible to prevent the oscillation. This is commonly called loop isolation. In any case these filter elements must have very pronounced characteristics, since the attenuation of the filter elements must always exceed, at a given frequency, the gains of the amplifiers at this frequency, which implies that these elements are expensive. In addition, even when the attenuation is sufficient, the impedance matching is often poor outside the useful passbands of each amplifier, and this may be sufficient to cause oscillations, since certain amplifiers oscillate when they are poorly matched.
U.S. Pat. No. 1,743,691 describes the insertion of a blocking filter circuit having a resonant frequency situated as an interruption between the said passbands. This arrangement increases the filter performance, but the basic principle nevertheless remains the same, with conventional filters which must have high performances.